This invention relates to a signal processing device with a band splitter, and, in particular, to an improvement of this device to control the noise in an audio signal with a low ratio of signal to noise.
Generally, electromagnetic waves that carry FM stereophonic broadcasts include a subcarrier containing a sum signal and a difference signal which are used to separate the stereo components of the main channel. The sum signal results from adding left and right signals from an audio source. The difference signal results from subtracting one of the left and right signals from the other. The sum and difference signals are both used to generate a broadcast transmission signal by modulating a carrier signal. The modulated br. oadcast transmission signal carries the sum signal in an FM subcarrier and the difference signal in an amplitude-modulated ("AM") subcarrier. The AM subcarrier is a suppressed carrier double sideband ("SCDSB") wave.
A subcarrier of 38 kHz is required at a receiver to permit the difference signal to demodulate the SCDSB wave. Therefore broadcasting stations transmit a pilot subcarrier of 19 kHz. In the receiver, the pilot subcarrier signal is doubled in frequency to 38 kHz, which is coherent in phase with the AM and FM subcarrier signals.
Prior-art methods of controlling noise employ a signal processing device that includes a band splitter. Such a device improves the signal by eliminating noise in some frequency bands. However, it leaves unmodified the noise in other frequency bands of the improved signal. Thus the improvement in signal-to-noise ratio of the prior-art device is less than optimal.